Improve notification message list processing

- If a message contained in the inbox list is deleted or read while
  there are messages in the overflow list, restore the newest message
  from the overflow list so there are always 5 messages in the inbox
  list in that case.
- Use explicit methods instead of method overriding.
Fix typo causing a NPE.
Some more cleanup

- When the last message of the message list is cleared, clear the whole
  notification
- Compare whole message reference, not only UID.
Documentation and cleanup

- Add Javadoc to new methods and classes
- Get rid of magic number
Merge pull request #223 from maniac103/jb-notifications-followup

Some follow-up fixes for the JB notifications